1. Technical Field
The present invention relates to a method and apparatus for manufacturing an arc tube for a discharge lamp in which a cylindrical shroud glass tube is welded integrally with the outer periphery of an arc tube body having a discharge emitting portion and an inactive gas space regulated to have a negative pressure is formed around the arc tube body, and more particularly to a method and apparatus for welding a shroud glass tube in an arc tube for a discharge lamp in which after a primary welding step of primarily welding one end side of the shroud glass tube to one end side of an arc tube body inserted and provided on a inside, the other end side of the shroud glass tube is secondarily welded to the other end side of the arc tube body while air is discharged from the tube through an opening portion on the other end of the shroud glass tube, and an inactive gas is introduced to hold the inner part of the tube to produce a negative pressure.
2. Background
As shown in FIG. 7, an arc tube has such a structure that a cylindrical shroud glass tube 4 for shielding ultraviolet rays is welded integrally with a bar-shaped arc tube body 2 including a closed glass bulb 2a that is a discharge emitting portion in the middle in a longitudinal direction. The closed glass bulb 2a is covered with the shroud glass tube 4 as is disclosed in Japanese patent publication JP-A-2002-163980. Ar gas is regulated to have a negative pressure, and is enclosed in a closed space 5 surrounding the arc tube body 2 formed by the shroud glass tube 4, thereby suppressing a devitrification phenomenon in the arc tube.
To manufacture the arc tube, the bar-shaped arc tube body 2 provided with the closed glass bulb 2a to be the discharge emitting portion in the central part of the longitudinal direction is fabricated. Next, the arc tube body 2 is inserted into the shroud glass tube 4 and one end side 4b of the shroud glass tube 4 is heated and molten, and is primarily welded (sealed) to one end side of the arc tube body 2 on the inside.
Subsequently, air is discharged from the inside of the shroud glass tube 4 through an opening portion on the other end of the tube 4, and Ar gas is introduced to hold the inner part of the tube 4 to have a negative pressure. At the same time, the other end side 4a is heated and molten, and is secondarily welded (shrink sealed) to the other end side of the arc tube body on the inside. Finally, the other end side of the shroud glass tube 4 is cut in a predetermined position if necessary.
In this arc tube, however, there is a disadvantage in that a shape in the secondary welding portion is not constant. At both of the primary and secondary welding steps, the shroud glass tube 4 is heated from sides respectively via a burner (heating means). The shroud glass tube 4 and the arc tube body 2 are held integrally and are welded (sealed) with the integral rotation of the shroud glass tube 4 and the arc tube body 2 with respect to the burner provided on the side at the primary welding step. At this time, it is necessary to discharge air from the inside of the tube through an opening portion on the other end of the shroud glass tube 4 and to introduce an Ar gas, thereby carrying out welding (shrink sealing) while holding the inside of the tube to have a negative pressure at the secondary welding step.
For at least this reason, it is impossible to weld both the shroud glass tube 4 and the arc tube body 2 with an integral rotation different from that of the primary welding step. Therefore, the welding is carried out such that both of the shroud glass tube 4 and the arc tube body 2 are fixed to the burner provided on the side.
Accordingly, the melting state of the shroud glass tube varies in a position opposed to the burner and the other position. As a result, there a first problem in that a shape and an adhesion in the secondary welding portion 4a becomes nonuniform in a circumferential direction, and the shape and adhesion of the secondary welding portion 4a might be varied for every arc tube which is manufactured, that is, a variation in the quality of the arc tube.
At the primary welding step and the secondary welding step, the support configurations of the shroud glass tube 4 and the arc tube body 2 are different from each other as described above. For this reason, the structures of the apparatus in the respective welding steps are different from each other, and the primary welding is carried out by an apparatus for the primary welding and the secondary welding is then carried out by an apparatus for the secondary welding. Thus, there is a second problem in that the size of equipment for the whole apparatus is correspondingly increased. Further, the workability is deteriorated.